Image-reading devices that simultaneously convey and read sheets are known in the art. These devices are sometimes required to read a sheet that is substantially thick and stiff, such as a plastic card. The torque required to convey such sheets (hereinafter referred to as “conveyance torque”) is greater than that required to convey sheets of normal paper. One conventional technique known in the art increases the electric current applied to a conveying motor in order to increase the motor's output torque, and decreases the electric current applied to the motor in order to decrease the output torque.